Different hemoglobins appear sequentially in the circulation during the development of many animals. These changes are brought about by differential expression of the various globin genes. The genes coding for the human non-alpha globin polypeptides are clustered together on a single chromosome. However, the mechanism by which different genes become active at different times is not well understood. We are using the human K562 cell line as well as normal erythroid cell cultures to study this problem. To characterize and quantitate the hemoglobin synthesis occurring naturally, or after induction, in these cells under various conditions, we are using gel electrophoresis and fluorography as well as sensitive and specific radioimmunoassays for hemoglobins A and F that we have developed. Similar studies of the globin mRNA produced by these cells, and of the effect of induction of hemoglobin synthesis on the structure and arrangement of the globin genes are being carried out using cDNA hybridization. Through these studies we are describing in detail some aspects of the process of erythroid differentiation in culture.